This invention relates to automotive tire pressure sensors. More particularly, this invention relates to a method and system for monitoring internal tire pressure of vehicles using an externally mounted sensor.
Tire pressure sensor systems are known and are commonly used to monitor the internal air pressure in individual pneumatic tires of a vehicle and to provide a warning signal to the driver whenever the internal air pressure in one or more of the vehicle tires is dangerously low or high. The warning signal is typically generated by an r.f. signal generator controlled by a microprocessor connected to the tire pressure sensor whenever the internal tire pressure measured by the sensor lies outside a predetermined normal operating range. This r.f. signal is transmitted to a vehicle-mounted receiver, which uses the warning signal to alert the driver either visually (by activating a warning lamp or display) or audibly (by activating an audible alarm) or both.
Known tire pressure systems typically employ one of two basic design configurations. The first type of design configuration employs an internal arrangement in which all components comprising the signal transmitting portions of the tire pressure sensor system are positioned within the tire casing. In a typical installation of this type, the pressure sensor, r.f. generator, the microprocessor, and the D.C. battery power source are physically installed inside the pneumatic tire casing prior to inflation of the tire, usually by attaching these components to an inner surface of the wheel. Design configurations of this first type suffer from several disadvantages. Firstly, the installation of the system components within the tire casing is not simple, requires careful attention to the component location and mounting, and increases the manufacturing cost of the entire automobile. In addition, many vehicles use steel-belted tires, which interact in a detrimental manner with the r.f. signals generated internally of the tire. Further, when the battery or one of the other system components fails, replacement of the defective component requires that the affected tire be removed from the wheel before replacement can be done, which is costly and time consuming.
The second type of basic design configuration used for current tire pressure sensor systems employs components which are mounted in a housing which is threadably attached to the exposed outer end of the valve stem of the tire. The sensor system components include a spring-biassed slider element translatably mounted in a guide which is threadably attached to the tire valve stem. The slider is exposed to the internal gas pressure within the tire when the guide is attached to the valve stem. The slider typically includes one or more contact elements which can interact with circuit contact elements mounted at preselected points within the guide. As the internal gas pressure increases within the tire, the slider is translated in a first direction within the guide. If the internal gas pressure reaches a maximum threshold value, the contact elements adjacent one end engage the guide circuit contact elements, which activates an r.f. generator and causes a high pressure warning signal to be generated. As the internal gas pressure decreases, the slider is translated in the opposite direction. If the internal gas pressure reaches a minimum threshold value, the contact elements adjacent the other end of the slider engage the other guide circuit contact elements, which activates the r.f. generator and causes a low pressure warning signal to be generated.
While this design arrangement avoids the disadvantages noted above associated with internally mounted tire pressure monitoring systems, the accuracy of such external mount sensors is adversely affected by the centrifugal forces associated with the rotating wheel. In particular, as the angular velocity of the wheel changes, the centrifugal force acting on the slider element within the sensor also changes, causing additional force to act on the slider. Thus, the linear position of the slider within the guide is a function of both the internal tire gas pressure and the centrifugal force acting on the slider. As a result, the sensor can easily produce false warning signals, and can also fail to generate a valid warning signal. This problem is compounded by the fact that the sensor guide may not be mounted exactly radially of the wheel, but at an angle with respect to the radial direction. Consequently, it is difficult to design a sensor in such a manner as to compensate for the centrifugal force component acting on the sensor slider. Efforts to provide a simple yet accurate and durable tire pressure monitoring system using an external valve stem mounting configuration have not been successful to date.